(1) Field of the Invention
The present invention relates to an earthed circuit for an electric railway car and, in particular, to an earthed circuit adapted to prevent a noise current from flowing into rails through both a first axle by means of which an electric circuit for the car is connected to the ground and a second axle by means of which a car body and a box holding an electrical apparatus are connected to the ground, and from adversely affecting any signalling system or the like coupled to the rails, the noise current being induced by an A.C. noise voltage generated at the occurrence of the switching operation of a power semiconductor device due to stray capacitances between the electric circuit and the box and between the electric circuit and the car body.
(2) Prior Art
The earthed circuit for an electric railway car according to a prior art is shown in FIG. 1. In the drawing, numeral 1 designates a car body, numerals 2 and 2a designate axles (or wheels), numerals 3 and 3a designate respectively first and second earthing brushes which are attached respectively to the axles 2 and 2a, numeral 4 designates a rail, numeral 5 designates a main electric motor, numeral 6 designates a box holding an electrical apparatus provided in the car, such as a chopper control unit or the like, numeral 7 designates a filter reactor, numeral 8 designates a filter capacitor, numeral 9 designates a power switching element which may be a power semiconductor device such as a GTO (Gate Turn-off Thyristor) for the chopper control unit, numeral 10 designates a free-wheeling diode, and numeral 11 designates a pantograph. According to the earthed circuit in accordance with the prior art, the first earthing brush 3 for a return circuit for the electrical circuit through which the electric current I flows and the second earthing brush 3a adapted to ground the car body 1 and the box 6 are made independent and are separately attached to the different axles 2 and 2a with interconnection accordingly being made. This arrangement has been made because it is not desirable to make the earthed point for the car body 1 and the earthed point for the return circuit common, since a voltage drop .DELTA.V is caused due to the passage of an electric current between the first earthing brush 3 through which the electric current I flows and the axle 2. Namely, the earthing of the car body 1 and the holding box 6 may be made stable because they are connected to the same potential as the rail 4 through the axle 2a and the second earthing brush 3a and are so designed that they are not affected by the voltage drop .DELTA.V caused in the first brush 3 by the electric current I flow in the return circuit.
Since the conventional earthed circuit for an electric railway car has been constituted in the manner described above, there has been a problem in that no account has been taken of the high frequency noise currents that are generated in the electric circuit for the car and which flow through the stray capacitance between the electric circuit and the car body 1 and the holding box 6 to the second brush 3a for connecting the car body and the box to the ground, and then through the rail 4 to the first brush 3 connected to the return circuit for the electric circuit. It is to be noted in particular that since a very fast switching speed is employed in the power control units or chopper control units of recent design which incorporate a GTO (Gate Turn-off Thyristor) and an FET (Field Effect Transistor), noises having high frequencies in the band of 100 KHz-1 MHz are likely to be generated and even a slight stray capacitance shows a tendency to generate a noise current in the form of a large spike, thereby increasing the influence on equipment installed on the ground or at a station, such as signalling systems and the like. This problem has become critical.
Some additional explanation will be given below with respect to the problems inherent with the prior art earthed circuit. In FIG. 2 is shown the noise current which flows through the earthed circuit shown in FIG. 1. In FIG. 2, the items indicated by numerals 1 through 11 are identical to those used in FIG. 1. Numeral 12 shown by a dotted line designates a stray capacitance between the electrical circuit and the holding box 6 while numerals 13a and 13b designate stray capacitances between the electrical circuit and the car body 1. An arrow i.sub.N1 designates a noise current which flows through the stray capacitance 12 while an arrow i.sub.N2 designates a noise current which flows through the stray capacitances 13a and 13b. In FIG. 2, the potential or voltage at, for instance, a point A will be varied in a stepped manner. For this reason, pulsed noise currents i.sub.N1 and i.sub.N2 are caused to flow to the holding box 6 and the car body 1 from the electric conductors and the leads contained in the electrical circuit which are at the level of the potential at the point A through the stray capacitance 12 between them and the holding box 6 and the stray capacitances 13 and 13b between them and the car body 1. Such noise currents i.sub.N1, i.sub.N2 having a high frequency are caused to flow out of the filter capacitor 8 with which the car is equipped and to return thereto through the earthed circuit. Namely, there are two paths taken by the noise currents, one being for the noise current i.sub.N1 which flows through the stray capacitance 12, the holding box 6 and the car body 1, and then the second brush 3a, the axle 2a, the rail 4, the axle 2, the first brush 3 and the earthed terminal of the filter capacitor 8, while the other is for the noise current i.sub.N2 which flows from the supposed leads to the car body 1 through each of the stray capacitances 13a and 13b, thereafter taking the same path as that of the noise current i.sub.N1 and finally circulating back to the filter capacitor 8.
FIG. 3 indicates an example of actual measurement of the noise current which flows through the rail 4 in the manner mentioned above, this measurement having been carried out in conditions equivalent to those of an actual railway car. It is to be noted that a spike-shaped noise current having crest values ranging from 100 A to 200 A has been observed in synchronization with the switching operation of the power switching element 9, which may be a power semiconductor device. Further, FIG. 4 shows an example of actual measurement of the spike voltage between points B and C on the rail 4 shown in FIG. 2, this spike voltage having been induced by the noise current. The drawing shows the width of the pulse by enlarging the abscissa which represents time. According to this illustration, the voltage having abrupt and high spikes has a high order of frequencies such as approx. 1 MHz followed by approx. 100 KHz of the oscillating voltage.
In this way, if the noise current flows to the rail 4 through the axles 2a and 2, the signalling system (not shown) electrically coupled to the rail 4 and the system (not shown) which is installed on the ground or station to provide a signal current to the rail 4 may be adversely affected such as to produce erroneous operation or non-operability. Thus a serious situation may occur leading to the electric railway car itself being prevented from running.